This project seeks to determine how waking neurobehavioral functions (sleep propensity, cognitive performance, sleepiness, and mood) are affected by partial sleep deprivation (PSD), and the time at which the sleep period occurs. PSD has been identified as a serious public health problem being experienced daily by millions of people as a result of medical, psychiatric, and sleep disorders, as well as social demands (work, family, travel). In addition, due to work and other constraints, people are not only sleeping less, but also at various times across the twenty-four hour day, or at different circadian phases. Although highly prevalent, little scientific work has been done on the duration of sleep, at different circadian phases, and number of days of PSD that will lead to increased sleepiness, involuntary sleep episodes, performance failures--information that is essential to determining the level of PSD that could result in accidents (e.g., motor vehicle). In a preliminary study, we developed a protocol for studying PSD and demonstrated the sensitivity of a set of neurobehavioral measures to sleep restriction over 1 week. Further, we examined the effects of restricting the normal nocturnal sleep period to 4, 6, or 8 hours in duration. This study demonstrated significant reductions in neurobehavioral measures when sleep duration was decreased to either 6 or 4 hours. Utilizing this approach, a randomized control trial using independent groups was initiated to test the effects on waking function of the types of sleep restriction most commonly experienced by people, at various times across the twenty-four hour day. The purpose of this study is to determine the nature of the cumulative waking changes in neurobehavioral functions engendered by restricting sleep to 4 h, 6 h, and 8 h (control) per night for 10 consecutive nights, when the sleep loss is placed at different circadian phases. Following a 7-14 day period at home, during which time the subjects are monitored to ensure they have a stable sleep/wake cycle, subjects will live in the Inpatient Study Unit of the CRC for 15 consecutive nights (16 days), during which time their sleep and waking neurobehavioral functions are monitored. Following 2 baseline days of 8h sleep/night, subjects will be randomly assigned to one of the nine sleep restriction conditions (4,6, 8 h/24 hours, with waking occurring at 0330h, 1130h or 1930h) for the next 10 days, followed by 2 days of recovery sleep (10 h/night). Changes in sleep polysomnography and waking neurobehavioral functions at different circadian phases across the 10 day period of sleep restriction are compared among the nine groups.